The present invention relates to welding and in particular to welding metals having a surface oxide layer such as, for example, aluminium alloy.
The high strength and low weight of aluminium alloy has traditionally lead to its use in areas where such factors are critical, for example in the aerospace industry. More recently it has begun to be used in the automotive industry as the material for vehicle bodies.
Difficulties exist in the welding of aluminium alloy due to its inherent nature. The high thermal conductivity of aluminium alloy ranges from three to five times that of steel, with the result that significantly higher inputs of energy are required to achieve fusion. For the welding of thick sections preheating of the work pieces may be necessary. Aluminium and its alloys have a surface film of aluminium oxide which forms rapidly when the aluminium is exposed to the atmosphere. It has been necessary to remove this oxide layer before welding as it prevents fusion between the work pieces. Oxide removal operations are time consuming and require a high degree of work piece cleanliness to be maintained if subsequent welding operations are to be successful. Magnesium and its alloys are also beginning to be considered for automotive applications. Like aluminium, magnesium also forms a surface oxide layer when exposed to the atmosphere.
These inherent problems with welding aluminium, magnesium and their alloys have thus far restricted their use in the automotive industry to top of the range vehicles and have necessitated the use of less desirable alternative joining methods such as riveting and adhesives.